1. Field of the Invention
The present invention relates to a rotor housing for a rotary piston engine and, more specifically to a rotor housing for a rotary piston engine with an improved gas-sealing property. The present invention also relates to a method of manufacturing such rotor housing for a rotary piston engine.
2. Description of the Prior Art
A rotary piston engine typically has a triangular rotor with convex sides which rotates in a rotor housing whose internal cross-section presents an oval shape slightly constricted in the middle (epitrochoid). The rotor has apex seals mounted on the three corners. When the rotor rotates the apex seals continuously sweep along the wall of the housing so as to maintain each chamber defined between the rotor and the inner wall surface of the housing in an airtight condition. In operation, if the apex seals or the inner surface of the housing are deformed due to the frictional heat therebetween the apex seals can wear out partially, which in turn causes a decrease in the gas-sealing property between the apex seals and the inner surface of the housing and thus a drop in the pressure of the compressed fuel may occur in the chambers.
It has been known in the field of rotary piston engines to apply to the inner surface of the housing a hard plating such as a chrome plating in order to improve the wear-resistant property thereof, as for example is disclosed in Japanese Patent Publication No. 25841/1984. Further, a film containing non-adhesive flouroplastic with a low coefficient of friction may be applied on the surface of the hard plating so as to improve the sliding property between apex seals and the inner surface of the housing. U.S. patent application Ser. No. 886,380, which was filed with convention priority based on Japanese Patent Application No. 166,391/1985, teaches that the rotor housing is formed at the inner wall surface with a Cr-Mo plating. The inner wall of such a rotor housing possesses excellent heat and wear resistant properties.
The above constructional of the inner surface show a good initial fit and prevent pressure drop in chambers between the rotors and the inner surface of the housing effectively. More specifically, the gas sealing property deteriorates when the apex seals wear partially due to thermal load such as heat from the friction between the apex seals and the inner wall surface. One cause of such loss of the gas sealing is thermal deformation of the apex seals themselves during the initial running period of the engine can be avoided to a certain extent by forming a hard plating or a fluroplastic film on the inner surface of the housing to improve the initial fit between the inner surface and the apex seals, as described above.
It is, however, impossible for the rotary housing with the inner surface wall formed of a hard plating such as a chrome plating to avoid sufficiently the drop in compression pressure in the chambers which is caused both during normal driving condition and in a high-power engine which gives off large amounts of heat.
The mechanism of such pressure drop is as follows. The rotary housing is formed with through-holes for ignition plugs at a predetermined position. The areas of the inner surface of the housing, which surround the ends of the holes, are exposed to the hot combustion gasses as well as the heat of the friction with the apex seals, and the temperature of these areas increases to a higher level than any other portion of the inner surface. Hence, the areas surrounding the hole ends are thermally deformed to project slightly toward the inner side of the housing due to the thermal load produced during the engine operation, especialy the high-load operation. The apex seals sliding on the inner surface are scratched by the projections of the inner surface and become unevenly worn, since the projections are formed of a harder layer, such as chrome plating, than the apex seals are. Thus, the gas sealing property between the apex seals and the inner surface gradually deteriorates, thereby causing a drop of the compression pressure in the chambers.